Fuel assemblies for a nuclear reactor and in particular fuel assemblies for a light water nuclear reactor are generally constituted by a bundle of mutually parallel fuel rods that are retained by a framework which comprises in particular longitudinal guide tubes and transverse spacer grids.
The fuel rods may be constituted by a cladding of zirconium alloy into which pellets of fuel material are introduced. The guide tubes of the framework may also be constituted by zirconium tubes.
The production of zirconium tubes from blanks requires a plurality of successive cold-rolling operations that are each followed by a thermal annealing operation. Between each of the cold-rolling operations and the consecutive heat treatment, a degreasing and chemical pickling operation is carried out on the rolled tube.
Planar products, such as sheets and strips, are also produced from zirconium alloy and are also subject to a chemical pickling operation, as may also be the case for the semi-finished products which are used for their production.
The chemical pickling of components of zirconium alloy is carried out using a solution of hydrofluoric acid HF which contains a given proportion of nitric acid HNO3 that is used as a catalyst for the corrosion of zirconium using hydrofluoric acid, in accordance with the chemical reaction Zr+4HF→ZrF4+2H2.
The used pickling solution which is recovered in a storage container after the pickling operation principally comprises hydrofluoric acid, water and nitric acid and zirconium fluoride ZrF4, which is formed during the pickling operation.
Baths for pickling zirconium alloy can be processed to separate the water from the solution of residual products, such as ZrF4, which are generally then discharged.
EP-A-0 723 038 from the company Zircotube describes a method for recycling used pickling solutions, wherein:                part of the water contained in the used solution is evaporated under vacuum then condensed in order to obtain water which is slightly acidic and a concentrated acidic solution containing ZrF4 which represents almost 30% by volume of the used solution,        the concentrated acidic solution containing ZrF4 is processed by vacuum evaporation in a crystalliser in order to obtain crystals of ZrF4 and a purified concentrated acid solution, and        the slightly acidic water and the purified concentrated acidic solution are mixed in the desired proportions in order to obtain a reprocessed pickling solution.        
Following the evaporation applied to the concentrated acid solution which contains ZrF4, there remains, in the bottom of the crystallizer, a suspension or brine of ZrF4 crystals. These crystals can be separated from the aqueous solution through a filter, such as a filter press, before being discharged to a storage center.
Production units for components of zirconium alloy must therefore involve significant costs with regard to the separation and discharge in approved centers. A significant proportion of zirconium is lost and not reused.
Having an efficient and economical method for recycling zirconium from this ZrF4 would be doubly advantageous by limiting the losses of zirconium metal in the overall production process for components of zirconium alloy and reducing the costs connected with the discharge operation.
Various methods have been examined, without any being able to be used in an industrial application.                The reduction of tetrafluoride by calciothermy. This is a highly exothermic reduction which must be carried out explosively in an inert atmosphere in order to prevent the metal from becoming contaminated by oxygen and nitrogen. It is therefore dangerous in industrial quantities. The reaction is as follows:ZrF4+2Ca→Zr+2CaF2         Direct electrolysis of the tetrafluoride has been examined but it is necessary to provide an installation which is too complex.        Van Arkel reduction is a technique which is not suitable since the molecule of ZrF4 is too stable. The decomposition heat thereof would require a filament temperature which is greater than the melting point of zirconium.        Another method is calcination. It involves bringing the zirconium tetrafluoride to a high temperature. It then decomposes into zirconium releasing molecules of hydrofluoric acid and water. The successive stages of thermal decomposition of ZrF4 according to Pascal (New treatise of mineral chemistry, volume IX, Ed. Masson, 1963, p. 519) are set out below. They are accompanied by the successive release of water vapour, HF gas and finally ZrF4 in gaseous form.        

There is currently no method which allows the zirconium contained in baths for pickling zirconium alloys to be effectively recycled on an industrial scale, in particular under temperature conditions which are acceptable in industrial terms. The highly corrosive nature of the compounds present is a difficulty whose effect increases with the temperature.